Recycling of the Anhydro- N -Acetylmuramic Acid Derived from Cell Wall Murein Involves a Two-Step Conversion to N -Acetylglucosamine-Phosphate

Abstract

ABSTRACT Escherichia coli breaks down over 60% of the murein of its side wall and reuses the component amino acids to synthesize about 25% of the cell wall for the next generation. The amino sugars of the murein are also efficiently recycled. Here we show that the 1,6-anhydro- N -acetylmuramic acid (anhMurNAc) is returned to the biosynthetic pathway by conversion to N -acetylglucosamine-phosphate (GlcNAc-P). The sugar is first phosphorylated by a nhydro- N -acetyl m uramic acid k inase (AnmK), yielding MurNAc-P, and this is followed by action of an etherase which cleaves the bond between d -lactic acid and the N -acetylglucosamine moiety of MurNAc-P, yielding GlcNAc-P. The kinase gene has been identified by a reverse genetics method. The enzyme was overexpressed, purified, and characterized. The cell extract of an anmK deletion mutant totally lacked activity on anhMurNAc. Surprisingly, in the anmK mutant, anhMurNAc did not accumulate in the cytoplasm but instead was found in the medium, indicating that there was rapid efflux of free anhMurNAc.